When I used to comete, 2-3 days of complete rest was the norm prior to the date.

As long as you're fit, CV will decline after 3-4 days, but it will take quite a while to drop enough for you to be concerned about.

If it is the briefing you're preparing for, I would personally say rest and the briefing isn't that CV intensive (but it will be difficult if you're trying to do it with a strain on the other side of the coin).